how to add a column with string values to a data frame while still converting them to factors

I have 2 questions. In essence i am trying to create product
categories based on product description and have it as an additional
column of my dataframe. some products dont fit any category and i need
a list of them. i am having some trouble in this simple (for most)
task.
Could you please provide suggestions. Thank you.

avneet

Question 1)

I have a data frame

agm.data=read.xls("agm.xls")

i add a column to it by this and similar statements

agm.data$ProdCategory[agm.data$Product.Description=="PGX"|agm.data$Product.Description=="PGW"|agm.data$Product.Description=="PS"|agm.data$Product.Description=="PSX"]="Molded

Graphite"

if i do

agm.data$ProdCategory

i get:

  [1] "Extruded Graphite"   "Molded Graphite"     "Molded Graphite"
   [4] "Extruded Graphite"   "Extruded Graphite"   NA
   [7] "Molded Graphite"     "Extruded Graphite"   "Extruded Graphite"
  [10] "Extruded Graphite"   "Extruded Graphite"   "Extruded Graphite"
  [13] "Extruded Graphite"   "Porous"              "Iso-Molded Graphite"
.
.
.
.
.
[1222] "Extruded Graphite"   "Molded Graphite"     "Extruded Graphite"
[1225] "Extruded Graphite"   "Extruded Graphite"   NA
[1228] "Iso-Molded Graphite" "Extruded Graphite"   NA

if i check the class, i get

is(agm.data$ProdC)

[1] "character" "vector"

i want this to have..
[1] "factor"   "oldClass"

..like other columns have

Question 2)some values of agm.data$ProdCategory are NA. i want to find
corresponding values of agm.data$Product.Description so i give the
following command

agm.data$Product.Description[agm.data$ProdCategory=="NA"]

i get:

[1] <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA>
 [17] <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA>
.
.
.
.
.
.
[257] <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA>
[273] <NA> <NA> <NA> <NA> <NA>
165 Levels:  1.563x24 10x50x60 61x1.25 6x72 890S 8x6 9x7; 10x1.5 AGR ... YBDXX88

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