Do foreign workers increase a country's wealth?

In the last few years the number of foreign workers in Malaysia has grown exponentially. These are mainly low income domestic helpers, restaurant workers, and field labourers. I wonder if the intake of foreign workers add or diminish wealth to a country. In theory, the use of lowly paid foreign workers is supposed to help free up the local population to do higher paid and more value added work. On the other hand, foreign workers are busy transferring their pay out of the country. Surely it would be better for money to re-circulate in the local economy by not having any foreign workers at all. Would Malaysia be better off having no foreign workers? I don't have an answer; I am still thinking through this.