Although ictime itself is not an accounting software and does not create and manage real invoices, it does not provide data for doing so, and it also allows for charging work via creating a kind of pro-forma "account". You can add reference data from the real invoice you have created in your accouting software to the "account" in ictime.
All basic functionality in the reporting & accounting area will work without having to define teams and price lists for a project. You can also charge work log entries that are historic (created before ictime was installed). However, all data won't have a financial dimension in this case. You might want to create teams and pricelists for each project where you have work log data relevant for accounting (see Project Teams). To apply price lists to existing work logs, you would need to edit and save the respective work logs.