adding p&l streams

i have a group of time series of p&ls

BEDH05U05_PL <- xts(yyy[,21], order.by = index(BEDH05U05_C['::2004-03']),
frequency = NULL )
BEDH04U04_PL <- xts(zzz[,21], order.by = index(BEDH04U04_C['::2003-03']),
frequency = NULL )

the p&ls are for only partially overlapping times...

tail(BEDH05U05_PL )

          [,1]
2003-03-24 83.50
2003-03-25 83.50
2003-03-26 75.25
2003-03-27 75.25
2003-03-28 82.25
2003-03-31 82.25

tail(BEDH04U04_PL )

          [,1]
2004-03-24 56.25
2004-03-25 56.25
2004-03-26 55.50
2004-03-29 55.50
2004-03-30 54.25
2004-03-31 54.25

i can combine the time series into a larger time series using

combined_pl <- merge (BEDH05U05_PL, BEDH04U04_PL )

i am trying to then add these two columns into a cumulative p&l. i tried
using combined_pl [is.na(combined_pl )] <- 0  to remove the NA values but
this doesnt solve the problem since when i hit the last p&l value in
BEDH05U05_PL the remaining values are set to 0 since they are NA. is there a
way to set all NA values until the first numerical value to 0 and then all
NA values after the last numerical value to the last p&l? i wasnt able to
find a fill forward type of function.

thanks  - brian