可以不用强调句：
Twenty years has been past since I had joined the company.20年前我加入的这家公司.
I have been working for the company in the past 20 years.20年来我一直在这家公司工作.
Edit:加入公司最好别用enter,而是join.enter只代表走进去,也许是参观,也许是找人.
此句中,有ago表示过去一个时间点,没ago表示过去一段时间里.when为adv.时候,意思是“at what time”,是表示在某个时间点的.when做为conj.时候,有“during the time at which”,可以表示一段时间.当然when做为连词时候,还有“although”的意思,但这个意思不是你这个句子需要的.
正式的英语中,when是不能接强调句型或者是跟"be"连接的,这是语法错误.然而非正式的口语中是可以这么用的,所以才不错.
如果你非要使用非正式英语,而且还表示一段时间,那就这么写：
It was more than twenty years when I join the company.
这话正式英语写法应该这样：
It was more than twenty years ago at the time that I join the company.
或者
It was more than twenty years during the time that I join the company.
多说两句吧,再举一个不能跟be连接的句子,如：
A dilemma is when you don't know which way to turn.（直译：进退两难就是当你不知道该走那条路的时候）is when 这就是错误的,但口语交流中不错,大家都懂.
正确的写法是：A dilemma is a situation in which you don't know which way to turn.