Can I write over an iron-on t-shirt transfer?

I made my boyfriend a personalized t-shirt for christmas using an iron-on transfer. I intend to give him a pack of fabric pens to further customize the design, but after making the transfer I notice that it leaves a plasticy-sheen. Will fabric pens work on top of the transfer? And will they be permanent? OR should I use a normal permanent marker? I am confused and any help is greatly appreciated!