I keep my multiple color pens separately from my Rocketbook but you can always add a pen holder like this

https://smile.amazon.com/Rocketbook-Pen-Pencil-Holder-Station/dp/B079VVS2Z1/?_encoding=UTF8&pd_rd_w=sCsNp&content-id=amzn1.sym.9d39c3fa-c33a-4f7a-8d61-434712e0d436&pf_rd_p=9d39c3fa-c33a-4f7a-8d61-434712e0d436&pf_rd_r=KYXZDRRCT1JBKDBPE188&pd_rd_wg=cvYUE&pd_rd_r=ff3a58a8-22dc-4fe2-b5d6-42946fb18cd0&ref_=pd_gw_bmx_gp_20oqmewn

or similar generic types. I would place it at the top of the cover so it doesn't get in the way. Note: The thicker FriXion pens may be difficult to get into the elastic part. Have fun with it!