Hi, thanks for reaching out.
You can request specific clothing/armour additions in the Addition requests thread.
In terms of editing your own characters with custom clothing, yes, that is possible. You cannot draw new clothing options directly within Top Down Sprite Maker, but you can edit the contents of the sprite style ZIP and modify the sprite style in any way you see fit, including adding clothing assets. I have published a sprite style development guide and made a tutorial video to help you do just that.
Please let me know if you have any other questions :)