Thinking about this overnight.....the software as is is designed to be able to handle company expenses to be billed to clients....rather than personal expenses to be re-imbursed by the company of which some will be billable to clients.
In an ideal world you would have an expenses functionality which allowed the user to input the gross expense incurred, the (sales/VAT) tax paid, whether the tax was reclaimable the company, whether the (net of tax) expense was billable to the client (and which client), what mark up would be charged on the net expense as it was charged through to the client, whether the charge levied on the client was taxable (on the invoice).
This should easily integrate with the current system obviating the need for me and others to use one system for managing personal expenses and then having to re-enter all chargeable expenses into your system.