"should I directly assume that the productivity per sales can stay the same as the existing sales force"

No, you should never assume. You should consider “what are the criteria for that to be true” and then confirm it with the interviewer. 

In other words, productivity can stay the same if the “bottleneck” is the number of salespeople, but everything else will remain similar, more specifically:

• The effort to reach those targets is similar 
• Targets have the same probability to buy 
• Targets are likely to buy the same quantity 

I.e.: Salesforce productivity = # of commercial visits per worker * % of success * average $ sale per client

In many cases, salespeople focus on “high grade” quality targets: easy to visit, high propensity to buy, large purchases. As you increase your salesforce, the quality of the targets starts going down (further away, less interested, smaller, …).

One can assess this either by evaluation productivity of recent new salespeople, or by understanding the segmentation and your penetration / market share / reach within each segment. This of course probably depends on the information you have at this point.

Of course, you don't need to state all of this (unless asked). But you need to state: “if we expand the salesforce will the quality of the target clients remain the same, or is it likely to be lower?”

The interviewer may then just answer for you and say it's safe to assume that, or may prompt you to explain your thoughts.