In general, if you're eating your veggies as prescribed by whichever
diet you're on, you're likely getting enough fiber. My general rule is
"all the low-carb veggies I can eat" - which adds up to more than 20 g
carb/day, which is OK by me. YMMV.

As far as those periods of time during which you may be not eating as
many veggies as you should, if your diet is high in fat, you're
unlikely to have any digestive problems. In fact, you may even have
diarhhea as the fat over-lubricates everything, even with no fiber in
your diet.

The standard answer to adding fiber to your diet is psyllium husk. But
the pure stuff isn't very edible, IMO. More for an "emergency" than as
part of a diet. And the doctored-up versions are full of sugar and
still not very edible, so...

A better addition to your diet is flax meal. It has sort of a
nutty-taste, kinda like hazelnuts. All the carbs are from fiber. Plus
if you keep it refrigerated, you are also getting good omega-3 fats in
flax meal.

A good breakfast is to nuke 3 TB flax meal and 3 TB unflavored,
unsweetened milk-based protein powder in 2/3 cup water, then doctor the
hot "cereal" with a bit of cinnamon or vanilla, sweetener and cream.
This is especially nice in winter.

You can also use flax meal most anywhere you'd use eggs as it sort of
thickens up when added to water.