It doesn't matter that "now" and "to finish off" are introductory, what matters is that "to finish off" interrupts the sentence.
A phrase that does this is called a parenthetical expression.
You could just as easily have said
Now I'd like to add one last stroke
but you want to emphasize that you're finishing up whatever you're doing, so you add "to finish off". It might not agree with everybody's writing style, but it's OK. The standard way of setting off a parenthetical expression is to put commas before and after. So when you include this one, you get:
Now, to finish off, I'd like to add one last stroke.
Occasionally, parenthetical expression are set off in different ways. Sometimes people might actually use parentheses
Now (to finish off) I'd like to add one last stroke.
or even em dashes
Now -- to finish off -- I'd like to add one last stroke.
but commas are the most common style. Sometimes it depends on how you want the meaning of the expression integrated into the sentence.