The add glyph and add characters features are available as stated, but adding OpenType GSUB tables is not yet available, and no such claim has ever been made. It would be useful for some users, but maybe not enough to justify adding to the next version. We will have to wait and see.
The Times New Roman font includes GSUB features for Terminal forms so that $FBA4 (ۀ) is substituted with $FBA5(ـۀ) wherever appropriate (at the end of a word, I presume). MS VOLT must be used to add these tables, or you could try using the method described in the Adding OpenType Features thread. It will take considerable effort to learn how, as it would if the feature was available in FontCreator.
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