The current version of xCell is pre-trained, so you can't add anything.
Even in the version we are currently working on that can be trained you won't be able to just add a signature, but need a reference transcriptome that includes the cell type of interest.
Can I define new celltype using xCell with a list of genes? For example, define TRM cells with "CD8A, CD69, CD103, PDCD1 and TOP2A"