Hi Jessie,
You can do this, but it requires being prepared for it ahead of time (since 3dDeconvolve will have to save an extra complete output for it).
Using 3dDeconvolve -xsave, it will save a ".xsave" file with various matrices and regressors. Then it is possible to run 3dDeconvolve again using -xrestore and passing that file in. You can then carry out new -glt tests.
But note that this only helps when you plan to do it, since it requires adding -xsave to the command.
- rick