This is automatic.

All you need to do is specify selling prices, in quotes and sales orders, in the units appropriate to the customer; and buying prices, in material supplier records and purchase orders, in the units appropriate to the supplier.

Providing suitable conversions have been defined, the system will convert between the currencies as necessary. So, for example, if you buy your widgets in US dollars and sell them in pounds sterling; the system will convert US dollars to the equivalent sterling price whenever you create a quote or a sales order for widgets, automatically.