By Impac ascii format I assume you mean the Mosaiq RTP-Connect format?
The only published, free software that I am aware of capable of
reading/writing to this format is this Ruby library:
Combining this with the Ruby DICOM library, you can create an RTPLAN
DICOM file from a RTP-connect ascii text file. However, it will
require you to figure out all the transformations, which although it
is probably not extremely hard, it is not quite non-trivial either.
I might be persuaded to implement a method which converts the
RTP-connect file to DICOM (I am the creator of these libraries).
Please contact me in private if you are interested in pursuing this.