Subjoin, v. t. [imp. & p. p. Subjoined; p. pr. & vb. n. Subjoining.]
Etymology: [Cf. OF. subjoindre, L. subjungere. See Sub-, and Join, and cf. Subjective.]
To add after something else has been said or written; to ANNEX; as, to subjoin an argument or reason.
Syn. -- To add; annex; join; unite.
Shades of meaning
Fernald has no entry of its own for subjoin; he lists it under add.
Where he writes about it:
A speaker may amplify a discourse by a fuller treatment throughout than was originally planned, or he may append or subjoin certain remarks without change of what has gone before. — from add