The disputed portion of the text of Mark 3:14 appears also in Luke 6:13 with the identical wording:
καὶ ὅτε ἐγένετο ἡμέρα προσεφώνησεν τοὺς μαθητὰς αὐτοῦ καὶ ἐκλεξάμενος
ἀπ' αὐτῶν δώδεκα οὓς καὶ ἀποστόλους ὠνόμασεν
And when day was-come, he called-[unto-him] his disciples. And
having-chosen from them twelve, whom also he named apostles
The rest of the Markan passage, though is not identical, most notably not using ἐκλεξάμενος but rather ἐποίησεν. Now, ἐποίησεν is a versatile word used approximately a kazillion times in Greek, primarily with the sense of "to make" or "to do". I don't see any clear cut examples of it ever being used to mean "appoint".
According to Liddle and Scott's Lexicon, the first usage with a sense related to "appoint" is from this passage by Isocrates:
(39) Παραλαβοῦσα γὰρ τοὺς Ἕλληνας ἀνόμως ζῶντας καὶ σποράδην
οἰκοῦντας, καὶ τοὺς μὲν ὑπὸ δυναστειῶν ὑβριζομένους, τοὺς δὲ δι’
ἀναρχίαν ἀπολλυμέ- νους, καὶ τούτων τῶν κακῶν αὐτοὺς ἀπήλλαξεν, τῶν
μὲν κυρία γενομένη, τοῖς δ’ αὑτὴν παράδειγμα ποιήσασα· πρώτη @1 (5)
γὰρ καὶ νόμους ἔθετο καὶ πολιτείαν κατεστήσατο.
The translation is given here:
Isoc. 4 39  For, finding the Hellenes living without laws and in
scattered abodes, some oppressed by tyrannies, others perishing
through anarchy, she delivered them from these evils by taking some
under her protection and by setting to others her own example; for she
was the first to lay down laws and establish a polity.1
1 The tradition is probably correct that Athens was the first city to
set her own house in order and so extended her influence over Greece.
The creation of a civilized state out of scattered villages is
attributed to King Theseus. See Isoc. 10.35; Isoc. 12.128 ff.. In
Isoc. 12.151-4, Isocrates maintains that certain features of the
Spartan constitution were borrowed from Athens.
Isocrates. Isocrates with an English Translation in three volumes, by
George Norlin, Ph.D., LL.D. Cambridge, MA, Harvard University Press;
London, William Heinemann Ltd. 1980.
In Hebrews 5:1 the author uses καθίσταται when he speaks of the appointment of priests to God's service:
Berean Literal Bible Heb 5:1 For every high priest, being taken from among
men, is appointed (καθίσταται) on behalf of men in things relating to
God, that he should offer both gifts and sacrifices for sins,
So I surmise that the disputed words were improperly taken from Luke 6:13. I further surmise that the Mark passage and the Luke passage are not intended to be congruent.
It appears to me that Mark is identifying the origin of the office of the twelve ("establishing a polity"):
BLB 1 Corinthians 15:5 and that He appeared to Cephas, and then to the
BLB Acts 1:17 He was one of our number and shared in our ministry.
But Luke is explaining the origin of the calling to apostleship.
In that light it isn't necessary to add the disputed words (though they don't hurt) or to coin a new usage of ἐποίησεν. Mark's passage is saying that Jesus "established a Twelve".