Working programmatically with the Bioconductor classes can be done using the following steps:

1. Create a new class inheriting from SummarizedExperiment:

setClass("BarcodedSummarizedExperiment",
   contains = "SummarizedExperiment",
   slots = c(barcode_id = "character", barcode_sequence = "character")
 )

2. Create a constructor function:

BarcodedSummarizedExperiment <- function(assays, rowRanges, colData, barcode_id, barcode_sequence){
   new("BarcodedSummarizedExperiment", 
       SummarizedExperiment(assays=assays, rowRanges=rowRanges, colData=colData),
       barcode_id = barcode_id,
       barcode_sequence = barcode_sequence
   )
}

3. Add the required methods to the class:

setGeneric("barcode_id", function(x) standardGeneric("barcode_id"))
setMethod("barcode_id", "BarcodedSummarizedExperiment", function(x) x@barcode_id )

4. Build an instance of the new class:

nrows <- 200
ncols <- 6
counts <- matrix(runif(nrows * ncols, 1, 1e4), nrows)
assays <- list(counts = counts)
rowRanges <- GRanges(    rep(c("chr1", "chr2"), c(50, 150)),
                         IRanges(floor(runif(200, 1e5, 1e6)), width=100),
                         strand=sample(c("+", "-"), 200, TRUE),
                         feature_id=sprintf("ID%03d", 1:200)
)
colData <- DataFrame(
                Treatment=rep(c("ChIP", "Input"), 3),
                row.names=LETTERS[1:6]
)

my_new_barcoded_experiment <- BarcodedSummarizedExperiment(
        assays = assays, 
        rowRanges = rowRanges, 
        colData = colData, 
        barcode_id = letters[1:6], 
        barcode_sequence = c("AT", "GC", "TA", "CG","GA", "TC") 
)

5. Call the new method:

barcode_id(my_new_barcoded_experiment)