I think it is going to be difficult at best.
Glance through the Third-Party drivers link that is on the Solaris HCL page:
http://www.sun.com/bigadmin/hcl/indexRes.html
Keep in mind that most of those links are to driver software
that is to be installed to Solaris-x86.
An example of what you may eventually get to ...
If you do successfully get such a card to work in a SPARC-based system,
it may only function AFTER the OS is fully booted, so it may not be usable as a primary interface.
The 3COM cards work very well in x86 architecture,
but it's a questionable exercise for SPARC systems.