Wayne,
This video walks you through it. Just tried it on mine and worked like a charm saving to memory. All programmed from the radio face.
That video merely shows how to set up a custom repeater offset in VFO mode. The video doesn't even attempt to show how to save the custom repeater offset to a channel memory.
The instructions on page 25 of the basic manual are the same, showing how to set up a custom offset in VFO mode, but not how to save it in a channel memory.
I'm convinced that it's impossible to store a custom repeater offset in an IC-9700 channel memory.
73,
Wayne Estes W9AE