Infix to postfix conversion
Canonically, we convert from infix to postfix form only expressions. However, you can enhance this by introducing the assignment operator = in addition to the standard four binary operators +, -, *, and /. Just define the conversion as: a = b  becomes  ab=. So for example, a = x + 5 becomes ax5+=. 

As for the negative numbers, you can parenthesize them (as you suggest). For example,  a = -b + 2 becomes a(-b)2+= or just separate each operand or operator by a single space, for example the previous postfix expression can be written as a -b 2 + =. This is another example: derv = -dy/dx + 5*(-x) becomes derv(-dy)dx/5(-x)*+= or derv -dy dx / 5 -x * + =.