I thing maybe don't understand what you are trying to do. Do you simply want to have a conference that requires the caller to enter a PIN? Or are you trying to send a caller to a conference that has a PIN but you want to send the PIN externally?
In case it's the latter you can do the following...
Create a conference locked by a PIN. In the logs below I used conf 3300@default and 1234 as the PIN. Then I called 3300 from user 1002:
2010-12-14 00:00:21.518045 [NOTICE] switch_channel.c:784 New Channel sofia/internal/1002@10.15.0.94 [09cad98f-6e12-45fa-9f01-e7d90b9ea771]
2010-12-14 00:00:21.520627 [INFO] mod_dialplan_xml.c:331 Processing Michael <1002>->3000 in context default
2010-12-14 00:00:21.530579 [NOTICE] mod_dptools.c:920 Channel [sofia/internal/1002@10.15.0.94] has been answered
Then I did "show channels" to get the uuid:
show channels
uuid,direction,created,created_epoch,name,state,cid_name,cid_num,ip_addr,dest,application,application_data,dialplan,context,read_codec,read_rate,read_bit_rate,write_codec,write_rate,write_bit_rate,secure,hostname,presence_id,presence_data,callstate,callee_name,callee_num,callee_direction,call_uuid
09cad98f-6e12-45fa-9f01-e7d90b9ea771,inbound,2010-12-14 00:00:21,1292313621,sofia/internal/1002@10.15.0.94,CS_EXECUTE,Michael,1002,10.15.30.10,3000,conference,3000-10.15.0.94@default+1234,XML,default,PCMU,8000,64000,PCMU,8000,64000,,ElToro2.FreePBXV3,,,ACTIVE,,,,
1 total.
Then I did "uuid_recv_dtmf <uuid> 1234" like this:
freeswitch@internal> uuid_recv_dtmf 09cad98f-6e12-45fa-9f01-e7d90b9ea771 1234
-ERR no reply
freeswitch@internal> 2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '0' to 'mute'
2010-12-14 00:00:41.505775 [INFO] switch_ivr_async.c:162 Digit parser mod_conference: Setting realm to conf
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '*' to 'deaf mute'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '9' to 'energy up'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '8' to 'energy equ'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '7' to 'energy dn'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '3' to 'vol talk up'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '2' to 'vol talk zero'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '1' to 'vol talk dn'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '6' to 'vol listen up'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '5' to 'vol listen zero'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '4' to 'vol listen dn'
2010-12-14 00:00:41.505775 [INFO] mod_conference.c:6485 sofia/internal/1002@10.15.0.94 binding '#' to 'hangup'
As you can see you do get "-ERR No Reply" but the operation itself works just fine.