Maybe this will work, I don't have that character so not sure.   Apply a magnet to the rfoot of the boot.  Under properties  add element to deform and select and rtoe and rshin of the boot .  Grab the mag zone and enlarge it  till it is about 1/2 way up the shin of the boot. Then you select the magnet and ytran it down till it lines up.  After that do the same for left boot. That way the boot should conform to the default character. Leave the magnets on the boots (instead of spawning morph targets)  so you can adjust later if they are off once posed.