If you are interested in volunteering you will need to research companies near where you live that use or support SAP (assuming you do not want to travel as you would not be getting paid). Your next bit would be to write a cover letter with your resume explaining who you are and that you would like to obtain work experience through volunteering. In this letter you would still need to "sell" yourself. That is, beyond having SAP BW/BI skills what other experience and personal attributes can you bring to the job to benefit them. You might have excellent data analysis skills, know some other software, etc.
What you will need to recognise if you may be working for free but some companies may not be able to use you or will not have the time to train you. Also, consultancies may not be able to use you on their client sites as they would need to go through client approval, training, etc. Effectively, that is what you are trying to get - on the job training and experience. You might find it hard to get an opportunity but it could be an interesting way to get your food through the door.
Take care in this approach that you are not mis-used (i.e. work for 12 months for free with the 'promise' of consideration for job opportunity).
So in short, research your location and identify companies to approach. Look for local job boards and recruiters as an options, etc.
Good luck with obtaining your experience