TAB
TAB has two behaviours. If there is a multi-line selection, then it is the same as INDENT. If there is no selection, or the selection is limited to a single line, then the current selection is replaced with a TAB character. By default, TAB has no KeyCombination, and is effectively disabled. To enable it, change INDENT's KeyCombination to null, and set TAB's KeyCombination to Key.TAB.noMods().
PS. Adding tab characters in the middle of a line is, IMHO, daft, because the formatting of the text becomes ambiguous. Columns will only line up correctly if everybody sticks to a standard tab width. There's a formal standard of 8 columns, which most developers ignore, preferring 4 columns or 2 columns.