I would like to use user-submitted data from one form field and add it to anther field, then submit the form to create a node that has my custom data. I'm having a hard time doing this. Any help is appreciated.
This is what the node api hooks are good for.
From the docs:
The node passed validation and is about to be saved. Modules may use this to make changes to the node before it is saved to the database.
You don't specify D6 or D7
hook_nodeapi() $op == 'presave'
You can modify values in your implementations of these hooks and they will be saved.
If you need the NID you have to use hook_node_insert, but you will have to save to the DB directly.