Felt tip pens can be refilled easily. There are water-based and alcohol-based inks available. The most well known is, of course, Copic. If the pen is sealed just pull the felt tip out carefully and use a syringe to refill through the tip hole.

I don't know about gel pens; I don't use them. However, many acrylic inks are thin enough to run easily through a cheap fountain pen. You can buy converters for this (expensive) or refill empty ink cartridges with a syringe. They make blunt needles for medical applications; makes it less dangerous.

Unfortunately, titanium dioxide white pigments are too thick for this. You can use them with an old-fashioned dunk nib though. Another good alternative for white over color is a good white gouache. I like Schmincke for this. White gouache, in fact, all gouaches, can be used with nibs as long as they're thin enough. There are also empty markers on the market, for acrylics and other paints/inks. Montana makes them in all shapes and sizes.

Try, experiment.

Happy new year.