The „minor eleven“ – notated as m7(11) or mostly abbreviated just m11 – is a minor 7th chord with an additional 4th. Because the 4th is a chord extension (it doesn't belong to the basic chord consisting of root – 3rd or sus2 or sus4 – 5th – 7th or 6th), it is written as 11 in the chord symbol. The 4 only occurs in sus4 chords.
A Cm11 has the notes C (1), Eb (m3), G (5), Bb (7) and F (11). In practise you will often omit the 5th, because you take it and go down a whole-step (2 frets) to get the sought-after 11.
There is actually no theoretical reason that should prevent you from extending a minor 7th chord with an 11. Just try if you like it and if you feel it fits the musical style or song.