Jstolaruk is probably right, replacing your 5/03 with an 04 is probably the easiest by far.
Potentially though, and I say potentially because I'm not at all sure, and coordinating the two may be a HUGE pain.
Prosoft MVI46-DFNT card to allow your 5/03 to communicate through ethernet, or some other device that does such, and then....
Prosoft AN-X2-AB-DHRIO to convert the Ethernet to DH+??
I currently use the AN-X2-AB-DHRIO with my 5/04 to convert DH+ to ethernet, but I dont see why you could'nt use it in the opposite fashion.
You probably would want to look into it more, thoroughly but its an idea if your completely opposed to just taking the easy route and buying a 5/04.
good luck