Can one preemptively increase skin looseness in donor area before hair transplants to make more FUT's possible in the future?

It is best to contact your doctor about pre operative care and instructions since each doctor has a different regimen.  Some doctors advocate scalp exercises to loosen the scalp.

This is the reason FUE procedures are gaining popularity. Even with scalp laxity exercises, there is the certainty of leaving the procedure with the linear scar. Consider having an FUE procedure, and there is no need to worry about scalp laxity.

Massaging the scalp firmly up and down can increase scalp laxity slightly in some patients if done persistently several times a day for several weeks. Provided there is enough scalp laxity, having a second Strip FUT does not increase the risk of a widened scar.Intra-operatively, some doctors use hyaluronidase injections to try and increase scalp laxity, especially if a would closure is tight.

It is believed that scalp exercises may improve scalp laxity, but in general its not reliable.  Patients who are concerned of a wider scar after a second FUT procedure sometimes consider other hair transplant techniques such as Robotic FUE/FUE which don't produce linear scars and have the potential to yield more follicular units.

Scalp massage prior to FUT is generally recommended to loosen the scalp tissue and allow for better closure post surgery.  It will not increase any density in the back of the scalp.  

You can do special exercises that I designed that will increase your scalp laxity over a three month period of time.  The stretching exercises must be performed for 30 minutes per day. Speak with your surgeon.