These phrases have the same meaning:
an existing X / X is existing / X exists
As do these:
a possible X / X is possible / X [sought word]
Is there a verb that corresponds to 'exists', but has the meaning 'is possible'? I would prefer a widely recognised word if one does exist, but will accept neologisms too.
In other words (suggested edit):
verb-ify "[sought word]: is possible" in the vein of "exists: is existing"
ONE CAN SKIP, AS OPTIONAL, ALL TEXT BELOW HERE. IT IDENTIFIES CHALLENGES FOUND IN ANSWERING THE QUESTION. MAY BE IT's HELPFUL.
To illustrate: (1) I went to a store and bought milk, or instead (2) I went to a concert (and did not buy milk). In fact: I went to a concert. (1) is merely possible, (2) is possible AND happened. Both have is-ness in a phase space, occupy a place there, but (2) ALSO corresponds to a dynamical history and so to a physical space.
(2) [sought word]-s AND exists. It is (selected) from (ex) it (nature). (1) [sought word]-s merely; it is present in a place (pos) and allows (ibil) an operation on it, e.g., taking a subset and this subset is nature (all that is).
To be possible means, translated, to be present in a place in a way that allows it to be selected. What exists is selected from this where this is that place, and by the way, is possible (it was in this). This exists is what we observe, it being (in general?) AND being selected.
No short verb can be formed from pos and ibil. For possibil is the original spelling of possible, I suspect. Either pos and ibil with other suffixes (e.g., posit, able) mean something different in the english language.
A) potentialiates (suggested by JB) Problem: what is potential exists (in sense of being selected from the phase space). Merely it exists in one form but can be further selected to also exist in another form. For example, a further excitation of a field or potential is a particle. Both exist. The field is there, in nature, like the particle. It exists, and is possible but not merely possible.
B) avails (suggested by SF). This suggests being present somewhere in a place. This place is not completely isolated off (unlike what is impossible, which also resides in a phase space of all impossibles, but nothing from that list ever interacts with what exists, due to its isolation; we say it's impossible, namely, what is present in that list is not given to be selected into existence.) What is available or avails here allows something done to it: such as have a subset extracted from this.
Problem? It suggests purpose, while "is possible" does not. Possibility of a thing is observer independent (and so is its existence).
Solution? We interpret the purposive aspect in a non teleological way. If nothing was possible then nothing would exist, and if nothing exists than nothing is possible after all, everything is impossible, including this impossibility. Something is possible therefore and something does exist. Possibility realizes a purpose merely in allowing existence to take place, no more no less, which is merely a universal consistency.
Maybe we are justified so in using "X avails" in this universal sense as a verb corresponding, in its effects, to "X is possible"?