Clear parent from your bone on metarig (alt+PAlt+P). Then in bone properties of your bone choose either basic.raw_copybasic.raw_copy
or basic.super_copybasic.super_copy
rig type, then enable Relink constraint, after it in "Parent:" field you can enter name of the bone of generated rig (Foot.l for example or DEF-foot.l) you want your bone to be child. If you want basic.pivotbasic.pivot
you can create new bone and parent it to your bone and then choose rig type basic.pivotbasic.pivot
as usual.