Again one of the great mysteries of what TMW sees as useful enhancements versus what they actually implement... :(
There is, AFAIK, still no way to add a symbol to the default table; all you can do is change the order in which the extant ones are used.
You can fake it by using text and any printable character available, though, which is rather inefficient as every symbol is then an object. But, you can get by.
I'll add (one of) my pet peeves in a similar vein as no builtin hatching patterns -- just why it's been 30 yr now since the Windows version with HG and still such rudimentary features aren't available is beyond ken... :(
