You can use the merge method.
I think you are using xts time-series objects. These handle the row names automatically. From help(merge.xts)
, there is a keyword argument join that you can use to control how the merge occurs. It defaults to 'outer'. Example:
dat = merge(assets[1:3,], FOC[,1:2], join='left')
> dat
ALD SFN TCO KIM CTX Close Adj.Close
2003-01-03 48.1 23.98 23.5 23 22.34 510 510
2003-01-06 48.1 23.98 23.5 23 22.34 NA NA
2003-01-07 48.1 23.98 23.5 23 22.34 518 518