The power now goes from a switch to a light. You want to take it from the
light to a new box with a switch and and outlet, and from the second switch
to a light. Is that correct?

In principle there is nothing wrong with that. The motion detector is part
of the first light and will no effect on a parallel connection. The
existing switch is certainly capable of handling 15a (although I have never
seen one not capable, it still might be prudent to check it).

Presumably the first switch goes to a box that has the first light mounted
on it. If so, it is a simple matter to put the wiring to the new box in
there.