simR simulate within subjects data from between-subjects design

Question

In my pilot study I applied a between-subjects design. For the follow-up study, a within-subjects manipulation seems more feasable. I want to conduct power analyses for the planned mixed-model analyses by using the SimR package. I wanted to use the pilot data for simulating a bigger sample. However, as I did not manipulate the condition within, but only between subjects before, I am not sure how to implement/consider this in my simulation, as this probably affects power considerably. Does anyone have advice how I can turn the between-factor into a within-factor for my simulations? My model is: Y = A*B*C + baseline + (1|subject)