You can add a Boundary when creating a Planar Surface but not after it has been created unfortunately
You can however use Grips to resize or move a Boundary around once it has been created - however if you edit the Polyline used to create a Planar surface boundary it effectively deletes the boundary and the Planar Surface disappears
If your Boundary starts out as a Linestring, or you convert it from a Polyline to a Linestring before creating the Planar Surface, you can use Grips to edit the Linestring, or Edit the Linestring or use the Insert Line Segment TML to add nodes to a Linestring etc to modify it - the Planar Surface is maintained throughout. You can also use Scale to make it bigger or smaller and retain its shape.
Alan