I've modified a few sails
myself with mixed results but always a good learning
experience. I've also had two sails
modified by professionals and found the cost reasonable in Costa Mesa, CA.
Unless the jib/headsails you've got to work with are high cut, it may be difficult to achieve the desired clew height for a functional staysail.
We do have a boomed staysail which we use with some frequency sheeted in tight while running--steadies the boat from rolling and when waves are smacking us in the aft quarter (pushing the bow upwind a bit) the tightly sheeted staysail introduces a little bit of lee helm
that is useful to keep us steadily pointing downwind.
We also have a spare staysail we can hank onto the twin forestays to use loosefooted as others here suggest would be best. Our jib
is way out in front on a bowsprit
so no interference
there for us.
Take a read of Port Townsend Sails document here on staysails : http://www.porttownsendsails.com/pdf/staysail.pdf
as this is a great overview of using staysails, how they are cut and so forth.