You should be able to accomplish that by doing the following:
Remove the 30 amp dual breaker from the panel and replace it with a
single 20 amp breaker.
if the line feeding the existing outlet has just two "hot" wires and a
ground lead, then connect the ground lead to the ground terminal on a
good quality duplex 120 volt receptical you replace the 220 volt
receptical with using one of the two "hot" wires for the 120 volt feed
from the new breaker, and the other for the neutral return, tagging both
ends of that second wire with white tape.
If the line feeding the existing outlet had four wires, then one will
already likely be white and connected to neutral in the breaker box, so
use that for the neutral return and use just one of the "hot" wires, for
the 120 volt feed from the new breaker, capping off the other "hot" wire
at both ends.
If the existing outlet was so old that it was fed by just two wires,
then you could install a GFCI outlet labled "No ground connected" and
use one of the two wires for the "hot" feed from the new breaker and the
other (tagged with white tape at both ends) for the neutral return.
Your local inspector may have his own opinions...
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