I would be very cautious about adding more than one parent to a person.
To do so has impacts on various reports - some unexpected. Descendant reports of the "blood" parent will show the step-child, and all of his descendants, multiple times in a descendant presentation, whether a graph, outline descendant report, or genealogy (register) report. (ie four wives: one natural and 3 step-mothers, will list the step child and all of his/her descendants FOUR TIMES). Also, the step will be included in a descendant chart report of an ancestor of a step-parent as if they were blood descendants of the step-ancestors.
Also, if one selects a blood-step parent duo as the "preferred" parents, it will be the ancestry of those parents that will show as ancestors of the step child in various ancestral charts and reports - which they are not.
I used to designate steps when I first got FTM as I thought that was neat. And then I started seeing the repurcussions in reports. Now I don't designate anyone as a step-child or step-parent. And, I never designate adoptive parents as preferred - as that will show ancestors of the adoptive parent as ancestors of the adopted person - when they are not his or her ancestors.