I would guess so. Yes, you can add multiple custom fields. The documentation doesn't describe what field names can contain, but you're probably fine if you stick to alphanumeric. For values:
"Merchant-defined data fields are not intended to and must not be used to capture personally identifying information. Accordingly, the merchant is prohibited from capturing, obtaining, and/or transmitting any personally identifying information in or by means of the merchant-defined data fields. Personally identifying information includes, but is not limited to, name, address, credit card number, social security number, driver's license number, state- issued identification number, passport number, and card verification numbers (CVV, CVC2, CVV2, CID, CVN). If Authorize.Net discovers that the merchant is capturing and/or transmitting personally identifying information by means of the merchant-defined data fields, whether or not intentionally, CyberSource will immediately suspend the merchant's account, which will result in a rejection of any and all transaction requests submitted by the merchant after the point of suspension."
What type of data are you trying to pass?