[dungeonguru]

So, here's a method that may work for you.

Step 1. Open or create a Custom tags.1st file for yourself and add something like this between the <document> tags that are in those files (You can look at the Community .1st files and see where there is a tags file you can use to copy):

  <group id="A2ZMulti" name="Multiattack">
   <value id="yes"      name="yes"/>
   <value id="no"      name="no"/>
 </group>

The above creates a custom tag group with two members:
A2ZMulti.yes
A2ZMulti.no

Whenever you create a .1st file you need to completely exit HL and reload it before anything shows up.

Step 2. On your base race or a copy of your base race that will be used instead of it bootstrap the xMultiatt ability.
On the Conditions button for xMultiatt set the timing to First/150 and the tag expression to

A2ZMulti.yes

This means that xMultiatt will only bootstrap if you have the A2ZMulti.yes tag assigned.

In my test example I have made a copy of the Elf race and given it an id of rA2ZElf.

Step 3. On your subrace that will have multiattack put an Eval Script that runs before Step 2 (First/100). This can be your script:

hero.childfound[rA2ZElf].field[rMultiatt].text = "I get more attacks."
perform hero.assign[A2ZMulti.yes]

In my example I created a subrace of rA2ZElf, so I know that I'm looking for the rA2ZElf race as my base race. I add my multiattack text and then I assign the custom tag I created in step 1.

I use A2Z as an ID for my example, but you can substitute your personal ID in your files.

This means that you need to replace almost all the races with copies of your own. It's not really that hard to copy/paste the bootstrap line into your .user files if you're willing. The eval scripting, not so much. But from a GM perspective I would only create PC versions of monsters on demand, not everything at once.