The quick answer is that this is technically possible at a software application level, but Opto22 has not implemented it at a firmware level. You would either have to implement it yourself or find someone who has already done it. Anyone out there done it? Maybe http://www.metronik.si/eng/index.php have some experience.
Opto22 does have a standard product for DNP over Ethernet, but not for 60870-5-101 over 485/232
http://www.opto22.com/site/pr_details.aspx?item=SNAP-DNP-ASDS&qs=100310061044,,,16,18&
If your project involved a large number of brains, Opto22 could implement 60870-5 as the two protocols are quite similar, but its important to understand that DNP is the standard for US influenced markets.
Just for general information, DNP and IES60870-5-101 are similar in functionality, but not compatible. Both were designed for point to point or multi-drop serial link architectures, but can work over radio, LAN, etc. Both protocols are used worldwide for electric power SCADA. DNP is dominant in North America, Australia, South Africa. IEC is required by legislation in some European countries. It is also common in the Middle East. In most of Asia and South America both are used almost equally. DNP has gained wide acceptance in non-electric power applications, where IEC is rarely used.