If you're talking Lithium NMC or other non-LFP 18650 cells, don't even try it. Their charge and discharge characteristics are too different.
LFP follows a FLA/AGM type profile with a peak voltage for 100% SoC charging followed by a "settling" to a notably lower voltage. Lithium NMC (3.6-3.7V) do not follow this. There is negligible voltage settling. This dissimilar behavior makes it essentially impossible to meaningfully combine them without likely damaging the LFP cells or risking fire with the NMC cells.
Better to DIY your own LFP.