Generating custom rainbow table?

I'd like to generate a SHA256 rainbow table with all alphanumeric characters for 10-character strings. However, I want to prepend a small string to the beginning of each string, let's say "bob". So I'd like to generate a rainbow table for:
bobaaaaaaaaaa
bobaaaaaaaaab
bobaaaaaaaaac
bobaaaaaaaaad
...and so on.
How can I do this? None of the existing rainbow table generators allows for adding a string to the beginning. Or is there? Or do I need to modify the source of one of the open source ones?
bobaaaaaaaaaa
bobaaaaaaaaab
bobaaaaaaaaac
bobaaaaaaaaad
...and so on.
How can I do this? None of the existing rainbow table generators allows for adding a string to the beginning. Or is there? Or do I need to modify the source of one of the open source ones?