The Argument
It is inquired whether the Greek phrase «κλητὸς ἀπόστολος» should be translated as “a called apostle” or “called to be an apostle.” The former, it is supposed, demonstrates a reality, while the latter a potentiality. That is to say, a “called apostle” is an apostle now, while one who is “called to be an apostle” is not now, but could be an apostle in the future. Consequently, “called to be an apostle” would seem to be an inaccurate translation if indeed Paul was an apostle at the time he wrote.
Examining the Grammar
First and foremost, «κλητὸς ἀπόστολος» can most certainly be translated as “a called apostle.” With that translation, Paul would be emphasizing his apostolic legitimacy “by the will of God,”1 unlike the “fake apostles” (ψευδαπόστολοι) he encounters throughout his missionary travels.2
That being said, “called to be an apostle” is a legitimate translation and it does not infringe on the fact of Paul’s present apostleship.
In the phrase “called to be an apostle,” the verbal “to be an apostle” functions as an adverbial infinitive modifying the quasi-verb “called.” I say “quasi-verb” because, while κλητὸς is technically an adjective (any lexicon would confirm so), many such adjectives ending in -τος (including κλητὸς) share the same meaning as a perfect passive participle. Indeed, they are referred to as verbal adjectives.
Herbert Weir Smyth wrote,3
Verbals in -τός, -τή, -τόν either (1) have the meaning of a perfect passive participle, as κρυπτός hidden, παιδευτός educated, or (2) express possibility, as νοητός thinkable, ὁρατός visible. Many have either signification, but some are passive only, as ποιητός done.
As such, the adverbial infinitive “to be an apostle” is timeless, because it does not function to tell us the time of Paul’s calling (i.e., when). Rather, it tells us why: “called to be an apostle.”
The only means by which we can determine the time of Paul’s calling (i.e., past, present, future) is if a conjugation of the verb “to be” preceded “called.” Read the following and appreciate the differences:

In the aforementioned examples, the conjugation of the verb “be” preceding “called” tells us when the calling occurred. However, because «κλητὸς» isn’t a Greek verb but an adjective, and Greek adjectives do not decline according to tense, the English translations rightfully lack the “was,” “is,” and “will be” preceding “called.” So, is there any way we can know when the calling occurred?
A Hint Remains
If we read a bit further in v. 1, notice how Paul writes «ἀφωρισμένος εἰς εὐαγγέλιον θεοῦ». ἀφωρισμένος is a perfect participle. The perfect participle indicates an action that occurred in the past and remains effective in the present.4 As Smyth wrote, κλητὸς has the meaning of a perfect participle. In other words, it is likely that Paul’s calling occurred in the past, coinciding with his separation for God’s gospel, both remaining effective in the present and into the future.
Too Long; Didn’t Read
Both translations are acceptable; neither translation precludes Paul’s present apostleship at the time he authored the epistle. “Called apostle” emphasizes that Paul is a legitimate apostle; he is not a “fake apostle.” “Called to be an apostle” rather emphasizes the reason (the “why”) of the vocation. Personally, I prefer the translation “a called apostle.”
Footnotes
1 cf. 1 Cor. 1:1
2 cf. 2 Cor. 11:13
3 p. 157, §472
4 e.g., «τῇ χάριτί...σεσῳσμένοι»—“saved by grace”; cf. Eph. 2:8
References
Smyth, Herbert Weir. A Greek Grammar for Colleges. New York: American Book, 1920.