If you want to add the same custom tag field to every track, you can do this with a simple tagger script. You can find this in the Options menu, under Scripting.
A tagger script with the next line:
$if($eq($get(#Helium Custom Field 1),),$set(#Helium Custom Field 1,default),)
will check for every file if the tag field #Helium Custom Field 1 exists. If so, nothing is changed. If not, the tag field is created and given the value default.
The $get() function is needed because of the hashtag and the spaces in the tag field’s name, calling the variable using %#Helium Custom Field 1% gives a scripting error.
And ofcourse you can expand this to set different values for different cases (for example for Various Artists releases, or based on genre), by replacing $set(#Helium Custom Field 1,default) in the line of code above with an appropriate $if() function.
Since the syntax for the tagger scripts is the same as for the naming script, you can find lots of examples in the documentation
Edit: if you have the option to clear existing tags active, you’ll have to add your custom tag field to the list of tags to preserve.