Personally, I am in Basis, but I can comment on what I have seen with our own FI-CO module leaders. We have had two in the time we have been running SAP and both of them have come from the accounting background and made very good FI-CO “sappers”. The only advice I could give is to try and get knowledge/training in the modules that closely link to FI-CO, e.g. PU (purchasing), assets and maybe some of the areas of SD (sales). Our current FI-CO person also manages the PU side of things, which seems logical.
Another point would be to try and CERTIFY in these areas, if this isn’t already being done through your training. This would only make you that much more marketable and valuable.