Recognition does not violate any law
If you first get the politics (of language right), the technique is (or becomes) secondary.
The constitutional recognition of one language as the working language of the Federal Government does not meant that making other languages working languages without constitutional amendment is illegitimate.
Adding more working languages can be done without any constitutional amendment (although a constitutional amendment will follow as a consequence).
The constitutional provision specifying Amharic as a federal working language (Art 5) is a declaratory provision; it is not a prohibitive provision. Consequently, the fact that it declares one language as a working language does not imply that making other languages working languages is disallowed or is otherwise illegal. the constitution
Once one adopts the right politics of linguistic justice, then the technique of effecting inclusive amendment is secondary, if not automatic. Coming to have more languages for the government in a multilingual country should be seen merely as a stride towards justice, towards expanding the horizons of equality. As such, there is no constitutional ban against it. If the justice of it is accepted (and there is no reason to believe otherwise), then:
a) we can not present constititional amendment as a hurdle for recognition of more languages. That is to say, constitutional amendment cannot be presented as a prerequisite, but as a consequence, of recognition.
b) we can take a political decision in favor of recognition and ground that in a parliamentary legislation pending formal constitutional amendment. (Technically, this contributes to what’s called informal constitutional amendment–which is widely accepted especially when it is to expand, not to restrict, the reach of rights.)
Invoking the need for constitutional amendment as a precondition for recognizing Afaan Oromoo (or any and all of the Ethiopian languages) as a Federal working language is a disingenuous pretext for the political reluctance to implement linguistic justice. It is a form of withholding the requiste political will to effect change.
More importantly, it is a mischievous (or a mistaken) interpretation of a declaratory constitutional provision as a prohibitive one in order to justify, rationalize, and mask the Ethiopian political elite’s disavowal of multilingualism.