First, take está entrando as a verb phrase: it's the way of expressing the progressive aspect of entrar (progressive = ongoing action). The core meaning is that of entrar.
Second, forget the literal translation; you have to think of the phrase as a whole. Me está entrando sueño means I'm getting sleepy, but neither the words nor the grammar will match.
The grammar here is parallel to that used for gustar and also for parecer and some other verbs that show an involuntary state or change of state. In Spanish the normal word order for these verbs is different from that of most phrases: indirect object, verb, subject or complement (e.g.: me entra sueño, nos gusta bailar, le parece incorrecto).
If you were to rearrange the phrase using the most common word order in Spanish, it would be Sueño me está entrando, which is ungrammatical because a bare noun cannot be used in this way. In English you could however say Sleep is getting into me, which is odd but readily understandable.
If you wanted to mimic the English phrase using the equivalence get = ponerse, you could also say Me estoy poniendo somnoliento, but that's an absolutely weird phrase, even if it's grammatically correct, and somnoliento is a formal word very unlike English sleepy.
Me está dando sueño is a common synonym. By itself it means the same as Me está entrando sueño, and the verb will always agree with sueño (i.e. it will be third person singular). It can also be used with other syntax, where sueño is not the subject but the direct object, and dar works as expected: Los medicamentos me están dando sueño.
There is no way you can use convertirse, in any conceivable phrase, to express the meaning of "I'm getting sleepy".