Yes, it is possible. There are several ways to produce your intended citation key, with or without a lookaround expression. One way is to replace a blank keywords field with 123. If the beginning of the field is adjacent to the end, then the field is blank.
[KEYWORDS:regex("^$","123")]
To use a lookaround expression on the whole field, first match the entire field with ^.* then add a positive or negative lookbehind to search for the string of interest. Remember to add .* after the target string or you will only get a match if the target is at the end of the field. This may not be very efficient, but it works.
This example generates the citation key yes if the keywords field contains mykeyword.
[KEYWORDS:regex("^.*$(?<=mykeyword.*)","yes")]
Here is a variation that generates alternative keys based on mutually exclusive regex expressions, and a blank key in case of no match.