Band-in-a-Box's StyleMaker creates styles in 3/4 or 4/4 only. However, consider that 6/8, 9/8, & 12/8 styles are justvariations of 3/4 or 4/4 - you can make and use styles in these time signatures to get the same effect.

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12/8: Band-in-a-Box considers 12/8 to be a variation of a 4/4 style (i.e. a 4/4 style with a triplet feel). To make a 12/8 style, just make a 4/4 style in a triplet feel. In other words, one bar of 12/8 is equivalent to one bar of 4/4 in a triplet feel. For an example, choose the "Country 12/8" style from the Styles menu | Choose from 24 'Built-in' Styles.
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6/8: This is similar to 12/8. Two bars of 6/8 make up one bar of 12/8, so 6/8 is done in 4/4 as well. Think of 6/8 as 2 beats of 4/4 in a triplet feel. For an example of this, go to the Styles menu | Choose from 24 'Built-in' Styles, and select the "Irish" style. You can set the number of beats per bar to 2 in the Edit | Settings for Current Bar dialog (F5).
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9/8: This is like a waltz (3/4) style in a triplet feel.
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5/4, 6/4, 7/4 etc.: Songs in time signatures like 5/4 and 7/4 can be made. Just pick a base time signature of 3/4 or 4/4, then for the song, select the time signature of 5/4 or 7/4 from the Edit | Set Time Signature menu item. Note that for time signatures higher than 4/4, each bar of 5/4 etc. will be spread over two or more bars in Band-in-a-Box. For example, 5/4 alternates between a bar of 3 and a bar of 2.

You can use the Search function in the StylePicker to find styles written for a specific compound time signature.
Note: Styles Set 38 for Band-in-a-Box ("About Time") has many excellent styles with unusual time signatures such as 5/4, 7/8, 11/8, 14/8, and 19/16. These styles have been written so that, for example, a bar of 5/4 occupies exactly one bar in Band-in-a-Box, rather than being spread out over two bars. Styles Set 38 requires Band-in-a-Box Version 11 or higher.