Sounds like a good idea to me. I don't think Radio Shack sells such an
extension, but they probably still sell suitable connectors, so you can
make your own.

I'd think an 1/8" mono-to-stereo (or vice-versa) adapter would work fine,
considering you're not having to deal with stereo signals for antenna pick-up.
On the other hand (and this may be a large hand!), my experience tells me that
the typical 1/8" ext.ant. jack on most portables (if not all made currently)
will not see the whip antenna properly unless the ext. ant. jack is vacant;
i.e., you could keep the adapter in, and just unplug the antenna cable, but if
you leave the adapter in, the radio will see the adapter as the antenna, and
ignore the whip. I'm sure I could be wrong, but I know that just plugging in an
1/8" plug with NO wire on it (or coax) shorts out the whip. Not much of an
antenna in that case. I'd say that you should just unplug it when you need to,
and keep a few extra jacks on hand, to replace onboard in the event of the
stock jack breaking.
Feel free to shoot me down if I'm wrong, but I just wouldn't want to see
someone buy or make an adapter plug which essentially killed their whip
reception when plugged in WITHOUT the attending external antenna.
Linus