Hi, if it doesn't have to be GenomicFeature package, then there are plenty of other options. I'm usually using biomaRt R package.

res<-results(dds)

library(biomaRt)
mart <- useMart("ensembl", dataset = "mmusculus_gene_ensembl")

head(listAttributes(mart)) # here you can see which attributes are available

ensemble2gene <- getBM(attributes=c("external_gene_name","ensembl_gene_id"),
                       filters = "ensembl_gene_id",
                       values = rownames(res), 
                       mart = mart)

match(ensemble2gene$ensembl_gene_id, rownames(res))
res$ensembl_gene<-rownames(res)
res<-merge(as.data.frame(res), ensemble2gene, by.x="ensembl_gene", by.y="ensembl_gene_id", all.y=T)
head(res)

In this case, rownames of my results were ensembl_gene_id, but you can of course adjust it to the values you're having. Then if you need you can replace the rownames with your external_gene_name and delete the additional column, that was created after merging.

Of course, it doesn't fit if your target genome is not in the package, and it's the reason you're creating custom database.

Regards

Artem