[Avodah] "kula" for gerim of patrilineal descent?

[Miryam T Brand]

I heard that there is a teshuva by Rav Moshe Feinstein that the child 
of a Jewish father and non-Jewish mother can be encouraged to 
convert.  Does anyone know where I could find this teshuva?  What are 
the halachic sources for this approach?

Thank you,
Miryam Brand