In 7/8 you can beam eights in different ways. Let's say you want them for example like 3+2+2.In the Time Signature dialog box you click on Composite. Check "Use EDUs for Beat Duration".
Let "Beat Groups" be: 1, 1, 1
and "Beat Duration": 1536, 1024, 1024
(If you prefer another grouping than 3+2+2 you can change the numbers above. To beam all seven eights you set "Beat Groups" to 1 and "Beat Duration" to 3584.)
Click OK. Check "Use a Different Time Signature for Display" (first you have to expand the dialog box by clicking on Options) and choose an ordinary 7/8.
You may have to rebeam your music, by selecting it with the Mass mover tool and then click "Rebeam music" on the Mass edit menu.
/Anders Feldt