yes you can .. check out the source code using cvs or get the latest tarball if that fails to compile .. then go make your changes, compile and link .. directions are on developer pages.
Hi,
Why the Opensees designer team doesn't add the distributed mass matrix? it seems this is a simple work. Does it have any conflict with the other parts of the software?
it's up to the element writers. some do .. some don't. it can have an effect on some of the eigensolvers or transient analysis solution using central differenence and a diagonal solver.