I think there is no direct way to do this but you can get it to appear in your payslip by defining a salary rule which sums up all previous payslips and after that add current months salary calculation
e.g.
result = payslip.sum('GROSS', '2013-01-01', '2013-12-31') + GROSS (I haven't tested this but I have used sum function before and it works fine BUT it can sum up only payslips which status is "done" drafts are not included in this sum).
you can either hardcode the dates like in that example or give them as input values or for the latter date you can test if python's datetime object is available (haven't tested this with salary rules so can't be sure).
Another alternative is that you create your own module which shows payslip different way.
I believed I may have created a duplicate: http://help.openerp.com/question/5486/best-way-to-include-ytd-totals-on-payslip/