[R] A function that can modify an object? Or at least shows principles how to modify an object?

[Aldi]

Hi, If I have an R object UUU, where the second element is U2, based on 
"g" column of my.table

my.table of UUU is:
mmm         ggg   gindex   map       Info
aaa123       U1       1           1             1
aaa124       U1       1           2             1
bbb1378    U2       2           1             1
bbb8888    U2       2           2             0
bbb1389    U2       2           3             1
ccc2222      Z3       3           1             1
ccc33333    Z3       3           2             0
ccc4444      Z3       3           3             1
ccc55555    Z3       3           4             0

Before (see Before) the analysis I had dropped those "mmm" rows from 
my.table of UUU that had Info==0, thus not included in the analysis.
After (see After) analysis found that I need to add bbb8888 (in the 
second position, as defined by map) for U2, ccc3333 and ccc5555 for Z3 
and s.o.
The bbb8888, does not have any info in general, but a collaborator is 
saying that he needs the bbb8888 for documentation in the UUU object.
A solution is to rerun the analysis, but the list is very long, the 
analysis will take 1 day. Is there any ready function that can modify an 
object? Or at least shows principles how to modify an object?
I have labeled elements of U2 as (e) which needs an NA in the second 
position; (d) needs a name bbb8888 as a second column and as a second 
row and correspondingly in its rows and columns use value 1.0; NO change 
for (c); adding 0.0 in the second position for (b); and NO change for 
(a). (see After) Thank you in advance, Aldi

Before:
=======
 > UUU[2]
$U2
$U1$scores (e)
[1]  -1.946707 -57.970488

$U2$cov (d)
                         bbb1378       bbb1379
bbb1378  1.10362564 -0.01222695
bbb1379 -0.01222695 26.88805020

$U2$n (c)
[1] 1802

$U1$maf   (b)
[1] 0.0002774695 0.0077691454

$U2$sey (a)
[1] 13.3867

After:
=====
 > UUU[2]
$U2
$U2$scores (e)
[1]  -1.946707 NA -57.970488

$U2$cov (d)
                         bbb1378  bbb8888     bbb1379
bbb1378  1.10362564  1.0  -0.01222695
bbb8888  1.0  1.0  1.0
bbb1379 -0.01222695  1.0  26.88805020

$U2$n (c)
[1] 1802

$U2$maf   (b)
[1] 0.0002774695  0.0  0.0077691454

$U2$sey (a)
[1] 13.3867

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