I'm not sure if I get you right, but I think "DESTDIR" is what you want.
Just make sure you have writing rights for the source and destination directory. Say destination is "usr/src/package-a" then just type
"make install DESTDIR=/usr/src/package-a"
then all things get installed in that directory. makepkg can then make a package of it, which you can install just as usual with installpkg.
My experience that it works often, but DESTDIR has to be defined in the Makefile, otherwise it does exactly nothing. But such packages are rare.