Add "Realign sequence(s) to other sequences" button to the Alignment Editor toolbar. It should be located to the right from the "Align sequence(s) to this alignment" button.
This button should be active when one or several sequences are selected in the alignment, all of them, none of them, their regions.
When the button is clicked, each of the selected sequence should be aligned to an alignment already made of non-selected sequences. The same alignment method as for "Align sequence(s) to this alignment" should be used (UGENE native or MAFFT depending on the tool availability, reuse the same functions call in the source code).