Asterisk registration into Broadsoft AS

Hi guys. I am trying to find an answer to this question: is it possible to do some SIP-header manipulation on Asterisk?

I am using Asterisk PBX 11.23.0, tying to register it into a Broadsoft AS. Registration is rejected due to Broadsoft AS is expecting Username :"+15874809450@ims.nghp.mnc221.mcc302.3gppnetwork.org" with domain and Asterisk is sending only Username :"+15874809450" during digest-challenge registration.

This is how my sip conf looks like

general]

udpbindaddr=0.0.0.0:5060
context=default
srvlookup=no
allowguest=no
defaultexpiry = 1800
alwaysauthreject=yes
register => +15874809450@ims.nghp.mnc221.mcc302.3gppnetwork.org:xxx:+15874809450@ims.nghp.mnc221.mcc302.3gppnetwork.org

[1001]
type=friend
host=dynamic
username=1001
secret=xxx
callerid=“Richard” <1001>
context=internal_ext
canreinvite=no

[mysipprovider-out]
type=friend
secret=xxx
username=+15874809450@ims.nghp.mnc221.mcc302.3gppnetwork.org
host=aaa.bbb.ccc.ddd
;fromuser=5874809450
allowexternaldomains=yes
;fromdomain=ims.nghp.mnc221.mcc302.3gppnetwork.org
canreinvite=no
insecure=invite,port
qualify=no
nat=yes
context=from-mysipprovider ; this section will be defined in extensions.conf

This would be pretty easy with a SBC in the middle, but this is not an option , at least for now. Any ideas?

Thanks.