Infusions are spells, and there are suggestions that new spells can be designed. Those suggestions were printed in Core, and not reprinted for each new spellcasting class (including artificer) was printed, but if, say, a favored soul or wu jen can research a new spell, then so too should an artificer be able to.
Of course, the real trick is convincing the DM that any particular invented spell is a good inclusion to the game. In practice, the suggestions that spellcasters can design new spells are rarely actually used—in most games, the DM is the arbiter of the allowed material, and the existence of that suggestion means nothing. If a player wants to add something to the game, no matter what it is, they can bring it up with the DM, but the DM is going to choose and usually won’t take the existence or absence of a statement like what you see for spells into consideration.