Currently there isn't an option to pass their registration info via Adaptive Payments Parallel Payments.
You may however want to look into Express Checkout Parallel payments as it will support customer registration details via the API and offers more flexibility. I've listed a sample request below.
This is an "advanced" request. Line item details are not required however is recommended to provide a better checkout experience.
USER=
PWD=
SIGNATURE=
METHOD=SetExpressCheckout
VERSION=98.0
RETURNURL=http://www.yoursite.com/mts/pp_nvp_quick_test.php
CANCELURL=http://www.yoursite.com/mts/pp_nvp_quick_test.php
PAYMENTREQUEST_0_PAYMENTACTION=Sale
PAYMENTREQUEST_0_SHIPTONAME=Daffy Duck
PAYMENTREQUEST_0_SHIPTOSTREET=123 looney lane
PAYMENTREQUEST_0_SHIPTOSTREET2= Apt 7
PAYMENTREQUEST_0_SHIPTOCITY=Miami
PAYMENTREQUEST_0_SHIPTOSTATE=FL
PAYMENTREQUEST_0_SHIPTOZIP=33010
PAYMENTREQUEST_0_SHIPTOCOUNTRYCODE=US
PAYMENTREQUEST_0_AMT=524.20
PAYMENTREQUEST_0_ITEMAMT=458.00
PAYMENTREQUEST_0_SHIPPINGAMT=20.00
PAYMENTREQUEST_0_TAXAMT=46.20
PAYMENTREQUEST_0_CURRENCYCODE=USD
PAYMENTREQUEST_0_DESC=test EC payment
PAYMENTREQUEST_0_PAYMENTREQUESTID=111
L_PAYMENTREQUEST_0_NAME0=Books
L_PAYMENTREQUEST_0_AMT0=154.00
L_PAYMENTREQUEST_0_NUMBER0=ABC123
L_PAYMENTREQUEST_0_QTY0=2
L_PAYMENTREQUEST_0_NAME1=CDs
L_PAYMENTREQUEST_0_AMT1=50.00
L_PAYMENTREQUEST_0_NUMBER1=BY-Z4736
L_PAYMENTREQUEST_0_QTY1=3
[email protected]
PAYMENTREQUEST_1_PAYMENTACTION=Sale
PAYMENTREQUEST_1_SHIPTONAME=Daffy Duck
PAYMENTREQUEST_1_SHIPTOSTREET=123 looney lane
PAYMENTREQUEST_1_SHIPTOSTREET2= Apt 7
PAYMENTREQUEST_1_SHIPTOCITY=Miami
PAYMENTREQUEST_1_SHIPTOSTATE=FL
PAYMENTREQUEST_1_SHIPTOZIP=33010
PAYMENTREQUEST_1_SHIPTOCOUNTRYCODE=US
PAYMENTREQUEST_1_AMT=494.20
PAYMENTREQUEST_1_ITEMAMT=428.00
PAYMENTREQUEST_1_SHIPPINGAMT=20.00
PAYMENTREQUEST_1_TAXAMT=46.20
PAYMENTREQUEST_1_CURRENCYCODE=USD
PAYMENTREQUEST_1_DESC=test EC payment
PAYMENTREQUEST_1_PAYMENTREQUESTID=222
L_PAYMENTREQUEST_1_NAME0=DVDs
L_PAYMENTREQUEST_1_AMT0=154.00
L_PAYMENTREQUEST_1_NUMBER0=ABC123
L_PAYMENTREQUEST_1_QTY0=2
L_PAYMENTREQUEST_1_NAME1=Computers
L_PAYMENTREQUEST_1_AMT1=40.00
L_PAYMENTREQUEST_1_NUMBER1=BY-Z4736
L_PAYMENTREQUEST_1_QTY1=3
[email protected]