I figured out how to do this with the standard SAP program RMDATIND. In case anyone else needs it, I've posted below a brief summary of the steps I used.

Create a new LSMW

Step 1. Maintain Object Attributes
Select the ‘Standard Batch/Direct Input’ Radio Button
Choose Object 0020 – ‘Material Master’
Method ‘0000’, Program ‘RMDATIND’, Program Type ‘D’ should be automatically filled in.
Save and Back

Step 2. Maintain Source Structures
Click create
Give a name and description to the source structure
I used: CONSUMPTION_DATA_SOURCE

Step 3. Maintain Source Fields
Create three fields for Material Number, Period and Consumption Qty.
I called them:

Field Name, Field Label, Field Length, Field Type
MATNR, Material Number, 15, C
PRIOD, Period, 6, N
MGVBR, Value, 4, N

Step 4. Maintain Structure Relations
Create a relationship between the source structure and the following target structures.
BGR00
BMM00
BMMH1
BMMH4

Step 5. Maintain Field Mapping and Conversion Rules
Create the following rules.
BMM00-TCODE = CONSTANT ‘MM02’
BMM00-MATNR = MOVE ‘CONSUMPTION_DATA_SOURCE-MATNR’
BMM00-WERKS = CONSTANT ‘1000’ (We only have one plant)
BMMH1-PERKZ = CONSTANT ‘M’
BMMH4-PRIOD = MOVE ‘CONSUMPTION_DATA_SOURCE-PRIOD’
BMMH4-MGVBR = MOVE CONSUMPTION_DATA_SOURCE-MGVBR

Step 6. Maintain Fixed Values, Translations, User-Defined Routines[SKIP]


Step 7. Specify Files
Create a new legacy data file on the PC in the following format. Filename.txt (I actually used tab delimited but comma delimited works too)

MATNR, PRIOD, MGVBR
123.456, 200802, 38
123.456, 200801, 12
123.456, 200712, 19
899.999, 200802, 102
899.999, 200801, 34
899.999, 200712, 87

Step 8. Assign Files
The system should automatically assign the file to the source structure created in step 2.

Step 9. Read Data
Click execute to read the data from the source file to the SAP LSMW file.

Step 10. Display Read Data
Displays the data read into the LSMW file.

Step 11. Convert Data
Click execute to run the conversion of the LSMW file to the format required by SAP.

Step 12. Display Converted Data.
Displays data in the LSMW file converted to the format required by SAP.

Step 13. Start Direct Input Program.
Choose ‘Program RMDATIND’
Choose ‘Using physical file name’ (the LSMW file should be here)
Choose the transactions per commit unit, desired lock mode etc.
Click execute to run.