Autocracies (forcibly) moved peoples around all the time if they thought it was to their benefit, going back to Nebuchadnezzar and beyond. Nothing ASB about it.
I suggest you focus on the 1930s, and not about moving people within one's borders, or about dumping people onto other countries. The point is about accepting 500,000 foreigners per year, for 8 years in a row. Think about it. It's just not happening.
To provide you with the basics for a comparison, the USA - a country numbering 131 million inhabitants in 1939 - welcomed just shy of 700,000 newcomers in the ten years 1930-1939. And there was a lot of opposition, a lot of pressure to reduce the numbers of those damn immigrants. This in a country that was born and built on immigration, a democracy, as progressive as you get them for that time.
That said, yeah, "if they thought it was to their benefit". We're talking about a nationalist regime here. Assuming they had the money, the capability and the vision to send people to good fertile land in the Ethiopian highlands (to be accompanied by significant protection against unfriendly locals, naturally, it's not as if the guerrilla was over in 1936) who do you think they'd reckon it was to their benefit to send?
Italian landless peasants, that's who. Not some Germans and Poles. Sending the latter, even in manageable, realistic numbers instead of the absurd millions, would be commented as: "Our boys conquered a colony for us at the price of their blood, and now Mussolini is stealing the best lands there and handing them out to foreigners". Mussolini was an autocrat, but he was keenly aware of public opinion.