Sorry for the delay, I ran into some issues I was trying to sort out. I'll get to that in a bit.

Ohki is correct, you could create a bulge map with the joint editor and that could be easier for you.

So we can go over how to do that real quick.

You don't have a whole lot of control over basic (traditional) bulge maps because they're based on the joints of the rig. If you open the joint editor (window>joint editor) with the arm selected, at the top (2nd row) you'll see a down arrow. Select Up-Down from that menu.

In the 4th box down you'll see Bulge Settings.

You can adjust the dials under bulge settings (right neg, right pos, left pos, left neg) to make the mesh around the joint bulge in or out depending on the shape you're looking for. It can get a bit tricky trying to remember which direction to use based on what position you're adjusting so just experiment with them. For the shirt you can adjust the right pos dial in the negative direction to pull the shirt closer to the figure's shoulder (when the shoulder is in the down position). Just be sure you have the shirt's shoulder selected and not the figure's shoulder. You'll need to move the figure's shoulder up and down to test the changes and it's easy to forget that you have to re-select the shirt or you'll be changing the bulge maps on the figure instead. You don't need the shirt conformed to the figure in order to adjust the bulge settings but it helps so that you can see how far to turn the dials before they begin intersecting the figure's mesh.

If you convert the figure to unimesh then you'll be able to paint bulges using weight maps, which gives much more control over how much the mesh bulges and where. Figure>Skinning Method>Poser unimesh, But the figure (in this case the conforming shirt) has to be converted to unimesh in order for this to work. If you have a pen tablet it will be easier due to the pressure sensitivity but you don't have to have one, it might just take a little longer. It's similar to using the morph brush

Once you've converted the shirt to unimesh, open the joint editor (if it's not already open) and go back to the Bulge Settings. Each map has an option to use weightmaps. When you check that box the paintbrush icon to the right will light up. That's a button to bring up the weight painting tool. Click it. You'll get this menu.

It functions similarly to the morph brush but instead of seeing a cluster of green and red dots you'll see a light grey bulls eye for your mouse and the green-red dots (vertices) will be static on the mesh. Green is 90-100% strength, red is medium strength and purple/black is almost nothing (these values are approximate). Subtract, Add, Smooth and Weight are the brush types. It will be set to Add by default. The brush shapes are the row of black dots across the center of the menu. The small dot furthest to the left will affect vertices one at a time with no falloff. The largest solid dot all the way to the right is basically the same thing but affects a much larger area, with no falloff. The 3 in the middle range in falloff zones. You can also adjust the radius of the brush with the Radius dial and the Magnitude determines how fast and far the mesh is moved, same as with the morph brush. Poser will remember the settings you have for each of the 4 brush types, which is helpful because you don't always want the same magnitude on each brush. I usually start with a 0.1 on magnitude. With the arm bent, and Add selected you can paint over the shoulder area like you would if you were using the morph brush. The mesh will push down closer to the figure's shoulder. Keep in mind that the dials in the Bulge Settings on the Joint Editor will still influence the strength of the bulge map you're painting, which means that you don't always need to paint a bulge map into the green for it to have a strong enough affect for the results you're looking for. From there it's just a matter of trial and error. Paint some, test the bend, adjust the bulge dials, repeat until you're happy with the results. Once you're satisfied you can close the joint editor and save a new copy of the clothing with your improved bulge maps. 

Hope this was descriptive enough to get you started.

I can explain how to set up a jcm in another post - I ran into some issues with it for M4 as it doesn't behave the same as with other Poser figures for some reason and I was trying to figure out why.