First, that player has to pay whatever the contract says he has to pay to opt out, before signing with another team (NBA or not NBA). Unless there's a specific opt-out clause (which specify an amount anyway), the player would be breaking a contract he signed, so he has a penalty to pay.
Then, regarding draft process, it depends on the players' age (and nationality, but I assume you are talking about non-americans). Quote from wikipedia:
Those who have reached the minimum eligibility age of 19 and meet the criteria for "international" players are automatically eligible if they meet any of the following criteria:
They are least 22 during the calendar year of the draft.
They have signed a contract with a professional basketball team outside of the NBA within the United States, and have played under that contract.
If they are past 22, technically it means they were undrafted, so yes, they would be able to sign.