Using Criteria To Update Fields In A Update Query

Ok, i have a question about update queries.

I have two tables (I'll call table 1 and table two for simplicity) and an update query. I want to get some data from table one to table two (via an update query). But in table two there is a field that isn't in table one but i want to add a value to that field via the query.

My question is, can i manually put into the query what data to add to a field instead of/aswell as using data from other tables.

I hope you understood my questions.

Cheers