There is no easy way to do this that I am aware of. If you search the forums, there have been a few people that suggested special, custom written scripts to pull text from the log file(s) to use to send alpha tags but there is no provision built in to DSD+ to do this.
Here is one post that I commented on that a user provided a possible option:
so I just got my feed back up after running cables and antennas and now I just need a way to get the alpha tags injected into the stream. has anybody been able to do this with DSD or some combo of DSD and other software?
forums.radioreference.com
Not sure if you are listening to a trunked radio system but I am guessing that you must at least be listening to something digital to require DSD+. Others may pipe up with more information but based what type of system you are scanning, another tool like SDRTrunk might be a better choice. A number of people have commented that SDRTrunk's audio is better than DSD+ also but I have not tried it yet. There may be other options also.