nawk worked. Just one bug that I've discovered in testing. The following input
modifyTimestamp: 20040719990632Z
modifiersName: uid=superadmin,dc=.
HsiaStaticAddrInfo: s999-232-127-159@xxx.net@0:0:0:0:0:0@999.23
2.127.159
HsiaStaticAddrInfo: s999-232-127-160@xxx.net@0:0:0:0:0:0@999.23
2.127.160
HsiaStaticAddrInfo: s999-232-127-161@xxx.net@0:0:0:0:0:0@999.23
2.127.161
HsiaStaticAddrInfo: s999-232-127-162@xxx.net@0:0:0:0:0:0@999.23
2.127.162
HsiaStaticAddrInfo: s999-232-127-158@foo.bar@1:2:3:4:5:6@999.232.127.158
HsiaStaticAddrInfo: s999-232-127-185@xxx.net@0:26:54:13

B:5C@9
16.232.127.185
HsiaStaticAddrInfo: s999-232-127-183@xxx.net@0

:87:39:AA:7A@91
6.232.127.183
HsiaStaticAddrInfo: s999-232-123-239@xxx.net@0:4:E2:2:CB:5A@999
.232.123.239
bctacHsiaServicePoint: Region 600
bctacHsiaServiceOption: SOHO@0@0@5@5@yes
createTimestamp: 19990929125757Z
generated the following output. Line with 999-232-127-185 is not merged.
modifyTimestamp: 20040712160632Z
modifiersName: uid=superadmin,dc=.
HsiaStaticAddrInfo: s999-232-127-159@xxx.net@0:0:0:0:0:0@999.23 2.127.159
HsiaStaticAddrInfo: s999-232-127-160@xxx.net@0:0:0:0:0:0@999.23 2.127.160
HsiaStaticAddrInfo: s999-232-127-161@xxx.net@0:0:0:0:0:0@999.23 2.127.161
HsiaStaticAddrInfo: s999-232-127-162@xxx.net@0:0:0:0:0:0@999.23 2.127.162
HsiaStaticAddrInfo: s999-232-127-158@foo.bar@1:2:3:4:5:6@999.232.127.158
HsiaStaticAddrInfo: s999-232-127-185@xxx.net@0:26:54:13

B:5C@9
16.232.127.185
HsiaStaticAddrInfo: s999-232-127-183@xxx.net@0

:87:39:AA:7A@91 6.232.127.183
HsiaStaticAddrInfo: s999-232-123-239@xxx.net@0:4:E2:2:CB:5A@999 .232.123.239
bctacHsiaServicePoint: Region 600
bctacHsiaServiceOption: SOHO@0@0@5@5@yes
createTimestamp: 19990929125757Z