You could try the following but I don't know if it will really work. There are probaby subtleties that I'm missing.

1. Extract the object files from the static library by doing this:

Code:
ar x libblah.a
This will create a number of object files, *.o.

2. Compile these object files into a shared library with gcc (or g++?):

Code:
gcc -shared -o libblah.so *.o
I think one possible problem is that if the original object files weren't compiled with the -fpic switch, the above will not work because code that goes into a shared library needs to be position-independent, which is what the -fpic switch does.