7/4 time

Create a custom 7/4 time signature (if you don't already have one). Select the time signature and go to its properties. There you can change the appearance to one of these examples.

(Picture from MuS 3.7)

However, an appearance of the type "3/4 + 4/4" would only be possible via the master palette and only as a graphic element. However, this leads to certain problems with the display.

I believe there's a 7/4 time signature in the Time Signatures palette. You may need to pull it out of the More list. However, I might have created it from the Create Time Signature button, which is also in the More list.

You can also create one by selecting 7/4 in the second screen of the New Score dialog.

A couple of tips:
"However, an appearance of the type "3/4 + 4/4" would only be possible via the master palette and only as a graphic element. However, this leads to certain problems with the display."

--I've done this several times, with time signatures such as "3/4=6/8", or "3/4 (5/4)", or "¢¢" (used for 4/2). The "actual" time signature, needed to provide the required number of beats in the measure, is rendered invisible, then the custom one is built, element by element, ("4", then "+", then "3", etc.) from the symbols master palette. Each element must be created by clicking on the first note or rest in the measure, then moving it into place.
--->Best to do this as one of the last steps of layout clean-up, as every subsequent layout adjustment might send it out of whack.
---->Also--the symbols palette has different sets of time signature elements for the different fonts--best to make sure they match, or they'll look all wonky.

In other words, going with a simple "7/4", or "4+3/4" is probably the best way to go, unless you absolutely need it otherwise. HOWEVER--when you create your custom signature, you can also adjust the beaming properties, so that eighth notes will automatically beam in groups of 8 and 6, for instance.