Short answer is that it is not a lot of fun.
If you have strings of length N in a string field:
Set up a target ascii converted block, e.g. 'stringascii'. Make it type signed word. It will need to be of length N/2 words.
Clear out the ascii target using SETNUMR with 0 value.
STRGETB your string into your ascii block.
If you need to reverse ascii (as I did for my HMI), you can publish using reverse bytes to an MHR register, or SWAPB to reverse the bytes in each word.
I iterated over this with a loop to convert a long string block into a long word block, and then just pointed my HMI to the correct offsets (N/2 for each string) in the MHR registers.
HTH,
C