I wouldn't hesitate, but I'd make certain that the outlet is in very good shape and has no issues of getting water where it needs to exit there before tying more dollars to the existing main.  There's a field we were working in last week where we installed a few new laterals to an existing 1970s vintage clay main, and in the same field we abandoned part of another line of the same age because it had a lousy silted in and very shallow outlet that somebody else had already tried to patch a blowout in.   We trenched new plastic to a new outlet with much better grade there, and dredged the road ditch of several yards of silt that were impeding flow.