Ok: Yes, this is possible. You need to reformat your internal memory card to a filesystem linux can run programs of and that maemo supports. This would e.g. be the ext3 file system. You can then link to applications there or link to entire directories.

But you need to make sure that you don't move anything there that's needed before the memory card is mounted. I haven't tested this, but you might run into such trouble when moving the entire /usr there.

When don't you just try it. You can always reflash the unit if something fails.