Without context it's hard to give you a specific answer. This term can mean a lot of things, and a number of translations would be appropriate depending on that context, including leaving the term out of the translation in some cases. For example, for ツイッター連携ソフト something like "software with Twitter integration" would be appropriate. For 連携ソフト that works with a device, simply "software for use with..." would work, and leaving the term out could work as well (e.g. <device> software). The term also sometimes simply refers to other software that work well with the main software, in which case terms like "compatible" or "works with", etc. There are other possibilities, as well. If you can provide more context perhaps we can provide a better answer.

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Note added at 6 hrs (2013-04-05 15:14:56 GMT)
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Considering your context, perhaps something like "component software" would fit your needs. Add-ons and middleware can both be considered components. I don't think these two are normally grouped together, so as long as you choose a terms that is reasonable, it should be okay since you are defining it.

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Note added at 12 days (2013-04-18 05:59:18 GMT)
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Like I mentioned before, you are defining the term, so it will not mean exactly what you want it to from the start. All of the following terms are other possibilities. They would likely be confusing without your definition, but it's a contract and the term is clearly defined, which nullifies this problem.

linking software
interlinking software
interfacing software
connecting software
interacting software

If you are determined to use a term that matches your definition, you can always use "associated and associating software," although I still don't think this is all that great since "associated software" could denote a very weak association that may not even have to do with the actual code. But again, it's defined in the contract, so you don't need to worry so much about the meaning of the term outside of the contract as long as it is reasonable and doesn't make you wonder why on earth they would have picked that word.