Perhaps I am misunderstanding the question? This is what pose files do. Just pose the character in a sitting position, at whatever location you want in the scene, and save the pose. When you apply the pose the character will come in at exactly the same position in the same pose. The only thing you have to watch out for is the position and rotations of the "BODY" element. This is important in two respects. 1). Poser does not save translations or rotations of the BODY element to a pose, so the x,y, and z translations and rotations of the BODY should be zero before you start posing the character, and you should position the character by translating the hip not the BODY. 2). When you apply the pose the translations and rotations of the BODY element should be zero, otherwise the pose translations will be added on top of those for the BODY.

If you don't want to manually zero the BODY's translations and rotations you can edit the pose file to do this automaticly, just add a body section, the format is like this:

{

version
{
number 4.01
}
thighLength 0.213434
//---Body section starts here---
actor BODY:2
{
channels
{
rotateY yrot
{
keys
{
k 0 0
}
}
//---More channels here---
translateZ ztran
{
keys
{
k 0 0
}
}
//---End of body section---
actor hip:2
{
channels

I have only shown the first and last channels above, but I hope you get the idea, you would have to include channels for each x.y,y,z, rotation and translation.

You could also implement the pose from a dial in the characters parameter pallet, via an ERC, but that is a little more complicated.