Ars Magica (HoH;TL, p.140) has three spells to make parts of a ghost’s equipment corporeal, but I couldn’t find a spell for the ghost itself. I feel compelled to point out that the Vision of Haunting Spirits (MuMe/Im 40, Ars p.150) is far better for making a ghost visible, which might be all that is needed. Also the Penetration total needed to cast these spells on even a mid-power ghost is very significant, so these transformation spells are not spells for junior Magi.
Blessing the Incorporeal Host with Earthly Form
Muto Mentem / Corpus 40, R: Eye, D: Sun, T: Individual
This spell grants an incorporeal spirit or ghost a corporeal human form.
(Base 25, +1 Eye, +2 Sun)
Blessing the Incorporeal Council with Earthly Form
Muto Mentem / Corpus 50, R: Touch, D: Sun, T: Room
This spell grants all the incorporeal spirit or ghosts within the targeted chamber a corporeal human form.
(Base 25, +1 Touch, +2 Sun, +2 Room)