(people) employed with work experience
Explanation:
It seems for me 中途社員 means 中途採用の社員, it means employee who joined the company with experience in some other places, not employed soon after the graduation. I suggest "(person/people) employed with work experience already", or even "employee with exprience/career also outside the company". I know it's not just a single word or two, rather explanation. However, when the cultural background is different, you should choose the one equivalent meanings over literally translated word(s). Just an advice. Take a moment to consider. Good luck.
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Note added at 23 hrs (2010-12-04 04:51:42 GMT)
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If the company asks for volunteers for resignation (under severe economic environment), this mid-career can be used in view of leaving the job in his/her mid-way in the career there. 中途採用の社員 are not necessarily experienced person; could be experienced in other field or even no relevant experience. Mid-career can be used, but often not very understandable or obscure even if you use it in the correct content. The worst case: mid- is middle, so not even very skilled yet.
Example sentence(s):- difference regarding salary between employees who have been worked since graduation and those who employed with work experiences
|  nekonote
Japan
Local time: 12:06
Native speaker of:  Japanese
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The translation workplace 
peer agreement (net): +3
