Yes it is possible, but how you do it I don't know. I had the same problem with my rv slideout. It was fixed by Steve Griffiths at Anglo American in Newport Gwent when the rv had it's annual end-to-end service.
Depends on the mechanism really.
If it's operated by cable and pulley system (which I believe is quite rare these days) the "squareness" of the slide can be adjusted by turnbuckles on the cable and the "side to side" position of the slide is usually controlled by guide blocks which are adjustable.
If its electric rack and pinion or hyraulic, you will probably have two cross members underneath the slide with hefty bolts going up into the underside of the slideout. These bolts go through slotted holes and by loosening ( NOT REMOVING!) these bolts, it may be possible to shunt the slide out in its aperture.
Be VERY carefull if you think about using a lever between the slide out wall and apperture edge as its very easy to damage the slide out seals.