You would need to have an RS-232 Interface on both ends. There are several options on the TI-99 end: if you are truly golden, you can get one of the very rare TI Hexbus Interface units for the 99/4A (either the original or the Michael Becker version). Then it would be a direct Hexbus to Hexbus connection without any additional hardware on either end.
You could do the same thing but insert RS-232 Hexbus peripherals at either end as well, if you were firmly wedded to the RS-232 mode and had the necessary hardware.
Lastly, you could use one of the sidecar devices for the TI that include an RS-232 connection (Captain's Wheel made one, as did CorComp (9900 Micro Expansion), Myarc (MPES-50), Mechatronics (CPS-99), and even TI). The TI sidecar shows up on Ebay a few times each year, as does the 9900 Micro Expansion, all others are much more difficult to find. Using this route, you could also get a TI PEB and insert one of the TI or third-party RS-232 cards into it for the TI end of the connection.
The connection does allow transfer of data both ways. I seem to remember that the CC-40 BASIC language is largely compatible with TI Extended BASIC as well, though I haven't done a command-for-command comparison to identify all differences yet.