All prices in a quotation must have the same currency. Hence if you want to add an item to your quote that you have a cost in a different currency you have to track down your trusty old calculator and convert the cost into the currency used in your quote.
You can however do all the calculations in one currency and issue the quotation in another currency.
Fist things first; select the currency and price list that you want to use for your calculations in the customer tab:
With the settings above, all figures used in the quote must be entered in USD. You might not even have USD available as a currency depending on your setup, so chose the currency that is applicable to your region.
When you're done with the calculation and you want to print the quote in a different currency, open up the Quotation information module and click on the Quotation tab (next to the Customer tab).
At the bottom right you'll find a drop down labeled Print currency and an Exchange rate field. From the drop down select the currency that you want to use for the sales price(s) of the quote and also enter the currency exchange rate in the corresponding field (in the sample below, from USD->BOB).
Now, open up the Summary module and click on the Totals tab.
As you can see from the image above, the Total incl. markup now appears twice, once in USD and once converted into BOB. Now enter the Quoted Price by rounding of the calculated sales price (in BOB, here 9.46) or by setting the gross profit margin.
Now if you print the quotation (using CTRL+SHIFT+P) the quoted price will be displayed in BOB.