I would Copy a measure which displays the time signature (the whole measure stack !) and Paste it (or Insert it). The time signature will be copied.

The solution here is Stack Selection - as Michel Herr points out.

To make a Stack Selection of a measure, with all the staves in the document, double-click the measure.

You can also make a Stack Selection that does not select all the staves.

To do so, first create a Staff Set of the staves you need to select.

Then, while viewing the Staff Set, double-click the measure.