[R] cubic spline curves

[Haibo Huang]

I have a question about how to generate monthly data
from quarterly data:

What I want to do is basically fits cubic spline
curves to the quarterly data to form continuous-time
approximations of the input series, and then generate
output series (monthly data) from the spline
approximations. 

SAS can do it using proc expand:
http://support.sas.com/rnd/app/examples/ets/expa/

Anyone know how to do it in R? Any input is
appreciated.

Best,
Ed