Remove punctuation only after single footnote citation?

Hi,

my footnote citations are configured in such a way that single citations always end with a fullstop (e.g. Watson 2013, 13.) and pultiple citations are connected with semi colon (e.g. Watson 2013, 13; Mayer 2009, 2.)

Sometimes after a single citation I would like to add not a second citation but some explanatory text. Like:

"Watson (2013), 13, this theoretical concept is also used by British scholars."

How can I remove the fullstop and replace with the a comma in just a single citation ?

Thanks!!