Interpolated class names and extend in LESS for BEM use

Question

I like using BEM for my css and I like to apply modifiers via the extend() function of LESS.

ex:

.btn{
  width: 100px;
  &--blue:extend(.btn){
    color: blue;
  }
}

1st question : Why can't I write it with the shortcut & like this ?

.btn{
  width: 100px;
  &--blue:extend(&){
    color: blue;
  }
}

2nd question : How can I call, in other less files, the generated classnames like :

.btn{
  width: 100px;
  &--blue:extend(.btn){
    color: blue;
  }
}

////

.myCustomButton{
  .btn--blue;
}

Thanks.