Is it possible to add more bars when creating a Pattern? With some of my songs a four bar groove isn't enough.

Pattern mode defaults to 4 measures, but you can set the LENGTH prior to recording to any length from 1 measure to 256 Measures.

Setting the length must be done prior to recording (naturally) Patterns cycle (loop)- so logically in order for the Instrument to "know" when to begin looping you must setup the LENGTH prior to recording. On the main PATTERN screen, move the CURSOR to highlight the LENGTH parameter

This "LENGTH" setting can be made per Phrase. So it is possible to have a looping 4 measure drum phrase on one track and a 16 measure bass phrase looping on another track. A "phrase" is defined as any recording done to a Pattern Track within a SECTION. Phrases in a Section can, indeed, be different Lengths as you may require. Shorter phrases cycle (loop) according to the Length you set just prior to recording it.
Press [F4] PATCH
Here you can view the 16 Phrases allowed per Section. Each is given a User Phrase number, each can be named, each also shows its Length.

If you have already recorded data into a PATTERN Phrase, then you can "lengthen" (append) or "shorten' (split) the Phrase to the number of measures you desire. This is accomplished using the PATTERN > PHRASE [JOB]
"APPEND Phrase" - this allows you to add additional measures to your phrase.
"SPLIT Phrase" - this allows you to divide what you have recorded and discard after a certain point.

If you need to add blank measures to the end of what you have recorded, you can simply transfer your Pattern Phrase to a blank SONG (starting at measure 001).
Then re-import the data plus the number of additional measures you require.

Say you recorded 4 measure, but now realize you really wanted to record a 16 measure Phrase.
You can use the JOB: PUT PHRASE TO SONG
Place it in a blank song starting at measure 001

Next use the JOB: GET PHRASE FROM SONG
But instead of just grabbing the original 4 measures, set it so you are importing 16 measures... your four plus 12 blank measures

And is it possible to record a pattern from my guitar using the a/d input?

No. The MOXF does not have the sampler - that would be the difference between the MOXF and the top-of-the-line Motif XF!!! With the Motif XF you can record external sound sources or even "resample" internal sounds, you can even record audio from your computer to any track you designated for audio. A track designated to hold an audio (Sample) Voice has 128 slots (Notes: C-2 thru G8) to place audio. Each Note across the keyboard could hold a different audio sample.

Hope that helps.