Primarily designed as an internal function for posterior.evolrate; however, can be used to graft a clade onto a receptor tree at the "sticky tip" labeled with "NA".
The donor clade needs to have a root edge, even if it is zero length.
References
Revell, L. J. (2012) phytools: An R package for phylogenetic comparative biology (and other things). Methods Ecol. Evol., 3, 217-223.