Multiple footnotes can be added consecutively in text. Use the 'Insert footnote' function, and just add each one after the last. To create a separator between each footnote, move your cursor between the footnote markers AFTER all the footnotes have been added, and then superscript the comma (if required).
Tips
When you add additional footnotes, add the footnote BEFORE adding the intervening comma as superscript, otherwise the second footnote will be superscripted to the first (i.e., set higher than the first).