nopeee, they are consider as Supreme Alpha, so they have the ability to impregnate everyone BUT the exact rules of the Omegaverse can vary greatly depending on the specific author or story though, if we're talking about logic, an Alpha cannot impregnate an Enigma because of the strict biological and dominance hierarchy of the universe, lemme yap about this because where else I can... so, Alphas are biologically designed to sire offspring, not carry them, they possess a "knot" and produce sperm to impregnate others.. Enigmas are the ultimate dominant gene, they are strictly "sires" (givers of genetic material) and possess male reproductive anatomy regardless of their external presentation, AND TWO SIRES CANNOT REPRODUCE, without a womb, eggs, or a receptive reproductive tract, an Alpha cannot physically implant a fetus inside an Enigma, but an Enigma can rewrite a biology, the defining trait of an Enigma is their ability to suppress and alter the gender dynamics of others, an Enigma can forcefully turn a dominant Alpha into an "Omega-like" state to impregnate them (the same could be said to Alpha and Beta)
an Alpha's pheromones and physical dominance are completely ineffective against an Enigma, because an Alpha cannot dominate an Enigma, they cannot trigger any submissive or receptive physical responses, think of it like "the top cannot be topped"
but of course, it completely depends on the author's own universe, in some stories, Figemas exist, and they are actually strong enough to top an Enigma !

in the rules of omegaverse that have enigmas, can they get pregnant by an alpha?!?