For each of your clients, you can define a default currency in the Currency field when creating a new client or when editing the Overview panel of an existing client.
When saving a price your client (under the prices tab), the default currency is the default currency of your client. But if your client works with multiple currencies, you can define prices in different currencies.
When you need to use another currency than the default one you defined for your client, you can simply define it in the currency field of the job (directly in the job creation popup, or by editing the Overview panel of existing jobs).
The system will use the rates you defined for your client for the currency.
If you're using LSP.expert V1
Normally, one client is associated to one currency, but it can happen that a client uses two different currencies.
In LSP.expert, you can only define one currency for a client, but there is a workaround to handle the "multi-currency clients".
You should simply create two different clients for the same client: one client that would be called "Client XYZ - EUR" for example, and another client that would be called "Client XYZ - USD".
When creating the job, you should then simply select the right "version" of the client depending on the currency you have to use for this job in particular.